Recovery of aromatics from multiple hydrocarbon streams



Jan. 14, 1969 G. F. AssELlN 3,422,163

RECOVERY OF AROMATICS FROM MULTIPLE HYDROCARBON STREAMS Filed OCT.. 2l,1965 United States Patent O 3,422,163 RECOVERY F AROMATlCS FROM MULTIPLEHYDROCARBON STREAMS George F. Asselin, Mount Prospect, lll., assigner toUniversal Oil Products Company, Des Plaines, Ill., a corporation ofDelaware Filed Oct. 21, 1965, Ser. No. 499,972

U.S. Cl. 260-674 11 Claims Int. Cl. C07c 7/10; C10g 21/00 ABSTRACT 0FTHE DISCLOSURE Solvent extraction process to which plural feedhydrocarbon streams are charged. A first feed, comprising aromatics andparaflins, is introduced to a lower intermediate point in the height ofa countercurrent solvent extraction column. A second feed, comprisingaromatics and naphthenes, is introduced to a middle intermediate pointof the extraction column. An aromatics-enriched solvent extract phase,withdrawn from the bottom of the extraction column, is subjected toextractive distillation to strip substantially all of the remainingnon-aromatics therefrom.

This invention relates to a process for the separation of hydrocarbonsfrom at least two hydrocarbon mixtures with the aid of a selectivesolvent for the purpose of separating and recovering the aromatichydrocarbon components of the mixtures. The process of the presentinvention serves to segregate the particular species of aromatichydrocarbons, such as benzene, toluene and/or C8 aromatics from otherclasses of hydrocarbons that are normally contained in petroleumdistillates, and utilizes a solvent which may be indefinitely recycledin the system, yields the desired hydrocarbon product in a state of highpurity and separates the same substantially in its entirety from thefeed stocks charged to the process.

The present invention is concerned with an improvement in the type ofseparation process wherein at least two mixtures of various classes ofhydrocarbons, the nonaromatic components of which in one mixture aremore readily stripped out of a solvent-aromatic mixture in an extractivestripper than the non-aromatic components of the other mixture,` areintroduced into an extraction zone at an intermediate pointthereof andare countercurrently contacted therein with a solvent selective foraromatic hydrocarbons, a lean solvent is introduced into an upper pointin the extraction zone, a rainate phase comprising substantially all ofthe nonaromatic hydrocarbons in the feed stocks is removed from theupper end portion of the extraction zone, an extract comprising thearomatic components of the feed stocks dissolved in the solvent phase isremoved from the lower end portion of the extraction zone and thearomatic solute is subsequently recovered by extractive stripping andfractionation of the extract phase.

In accordance with the present invention, said one mixture containingnon-aromatic components having a tendency to be more readily strippedout of the extract phase in an extractive stripper than the non-aromaticcomponents in the other mixture is introduced into the extraction zoneat a point closer to the withdrawal of extract from the zone than theother mixture in order to improve the aromatics purity and/ or yield asdescribed hereinafter.

In one of its embodiments, this invention relates to a process for theseparation and recovery of aromatic hydrocarbons from at least twohydrocarbon containing fluids having a solvent extraction step to forman extract phase and a raffinate phase and an extractive distillationstep to remove non-aromatic hydrocarbons from the 3,422,163 PatentedJan. 14, 1969 extract phase in which a substantial portion of thenonaromatic components of the first fluid have an increased volatilityin the extractive distillation step when compared to a substantialportion of the non-aromatic components of the other fluid whichcomprises: introducing a lean solvent into an upper intermediate pointin the height of an extraction zone; introducing the other fluid into amiddle intermediate point in the height of an extraction zone;introducing the first fluid into .a lower intermediate point in theheight of an extraction zone; countercurrently contacting the solventwith the fluids in the extraction Zone; removing a hydrocarbonreaflinate phase consisting essentially of non-aromatics yfrom an upperpoint of said zone; removing a solvent extract phase enriched inaromatics content from a lower point of said Zone; introducing theextract phase into an extractive distillation column and strippingessentially all of the non-aromatic hydrocarbons from the extract phase;and separating the aromatics from the solvent.

Solvent extraction processes for the recovery of aromatic hydrocarbonsfrom hydrocarbon mixtures have been known for many years. This isusually accomplished by methods such as extractive distillation oralternately by countercurrently contacting the feed with a solventselective for aromatics in the liquid phase within an extractor. Thepresent process relates to both of these methods. In general, thesolvents described hereinafter exhibit an increasing solubilityselectivity for hydrocarbons as the hydrocarbons degree of unsaturationincreases. Thus, the selectivity of a solvent is -greater for aromaticsthan naphthenes, greater for naphthenes than parai'lins, etc. Likewisein general a solvent exhibits an increasing solubility selectivity for agiven class of hydrocarbons as the number of carbon atoms per moleculedecreases. In the past it has been a common practice to introduce thefeed or feeds into an intermediate point in an extraction zone. In caseswherein the solvent is denser than the feed, the solvent is introducedinto an upper point in the extraction zone and the feed and solventbeing immiscible, countercurrently contact each other therein. The moreunsaturated feed cornponents tend to dissolve in the solvent phase to agreater extent than the more saturated feed components. However, acompletely clean separation of aromatics cannot be attained since asmall amount of the more saturated feed components will also dissolveinto the solvent phase. Therefore, the extract phase is sent to anextractive stripper in order to vaporize the more saturated feedcomponents and leave a solvent phase containing only aromatichydrocarbons. The stripper is able to remove the non-aromatic componentsof the extract phase because the presence of the solvent in the stripperhas less effect on the boiling point of the non-aromatic componentsdissolved in the solvent than it does on the aromatic compounds alsodissolved in the solvent. In general, the solvent tends to retain insolution the more soluble aromatic component even at temperaturesconsiderably above the normal boiling point of the aromatics alone. Thiseffect can be further emphasized by introducing additional lean solventinto the stripper along with the extract phase. However, the amount ofsuppression of boiling point of the aromatics is limited and whenprocessing charge stocks of wide boiling ranges it becomes impossible tovaporize the heavy non-aromatics without also vaporizing the lightaromatics. lf these heavier non-aromatics are not removed from theextract phase in the stripper they will ultimately reduce the purity ofand contaminate the heavier aromatics. Attempts have been made in theprior art to overcome this problem by methods such as the use of a lightparaffin backwash introduced into a point in the extraction zone betweenthe feed introduction and extract withdrawal to displace the heavier andmore diffi- ICC cult to separate non-aromatic components from theextract phase as shown in U.S. Patent No. 3,037,062. In many cases theuse of a light backwash material is not necessary since the feed is ofintermediate boiling range and although satisfactory aromatic purity andrecoveries can be made, nevertheless, the concentration of non-aromaticsin the aromatics is higher than desired. In these cases it may not beeconomically justified to buy the extra equipment. Likewise the feed maybe of such a narrow boiling range that a backwash stream is not veryeffective. For example, when using a feed stock composed of a reformateand a hydrotreated pyrol-ysis oil, both of which are of about the samenarrow boiling range, the present invention is employed to produce justas -good results as could be obtained by employing a backwash stream. Itis also possible to combine the present invention along with thebackwash step to further improve the process for the production ofaromatics.

It is an object of this invention to overcome the foregoing contaminantproblem in solvent extraction processes in a simple, inexpensive manner.

It is another object of this invention to use the more saturatednon-aromatic components of one feed to displace the more unsaturatednon-aromatic components of a second feed from the extract phase.

It is still another object of this invention to use the lower molecularweight non-aromatic components of one feed to displace the highermolecular weight non-aromatit` components of a second feed from theextract phase.

It is a further object of this invention to increase the yield and/ orpurity of recovered aromatics in a solvent extraction process.

These and other objects will become more apparent especially in thelight of the following detailed description.

Referring to the accompanying drawing, feed `#l is introduced into owconduit where it picks up a reux stream from a source describedhereinafter and flows into a lower intermediate point in extraction zone3. Preferably, feed `.iii-'1 is introduced into extractor 3 from about1A; to about 1/12 of the way along its height from the bottorn. Feed #2is introduced into flow conduit 4 where it ows into a middleintermediate point in extractor 3. Preferably, feed #2 is introducedinto extractor 3 from about 1A; to about 1/10 of the way along itsheight from the bottom. The distance between the lower intermediatepoint and the middle intermediate point is preferably at least 4% of thetotal height of extractor 3. Lean solvent from a source describedhereinafter is introduced into an upper intermediate point through flowconduit 6 whereupon the solvent ows downflow through extractor 3 andcountercurrently contacts the feeds which flow upow through extractor 3.The extractor contains contacting means such as bafe plates, sievedecks, rotary discs, packing, etc., to eiciently intermix the immisciblesolvent and feed phases. The extractor is maintained at an elevatedtemperature and pressure sufficient to maintain the solvent and thefeeds in the liquid phase. Since the solvent has a selectivity for thearomatics and the solvent is immiscible with the feeds, aromatics willpreferentially dissolve into the solvent phase. Thus, as the solventpasses downow through the upper portion of the extractor, the aromaticsconcentration therein gradually increases while as the feed hydrocarbonphase passes uptow through the extractor due to its lower density (thanthe solvent phase) the aromatics concentration therein graduallydecreases. When the hydrocarbon phase passes the upper intermediatepoint, it contains essentially no aromatics. This material is called theraffinate phase and is removed from extractor 3 through an upper pointwhere it passes into flow conduit 7. The raflinate may be fractionatedand/ or water-washed to remove small quantities of dissolved solvent.

The extract phase comprising solvent, substantially-all the aromatics inthe Ifeeds, and some non-aromatics is removed from a lower point inextractor 3 where it passes into ow conduit 8. The non-aromatics presentin the extract phase are derived from the non-aromatics present in thematerial flowing in flow conduit 5, which is the recycle stream in flowconduit 15 and feed #1. Feed #1 contains non-aromatics which are morereadily vaporized in the extractive stripper described hereinafter thanthe non-aromatics present in feed .#2. For example, a feed #l having thenon-aromatic portion rich in paraftins and a feed #2 having thenon-aromatics portion rich in naphthenes are employed as feeds and areintroduced into extractor 3 as described hereinabove. Paraflins are lessselectively absorbed by the solvent phase :but since their concentrationis higher in the hydrocarbon phase between ow conduits 4 and 5 in theheight of the extractor, there is suflicient driving force to allow theparaflins to displace the naphthenes from the solvent phase to thehydrocarbon phase. Therefore, the extract phase leaving extractor 3through ow conduit 8 contains predominantly parafns as the non-aromaticportion. These parafiins are easier to strip out of the extract inextractive stripper 11, described hereinafter, and will result in higheraromatic purities in the finished aromatic product. As another examplefeed #1 is lighter in molecular weight and lower in boiling point thanfeed '#2, In this case, light nonaromatic hydrocarbons are doublyeffective in displacing the 4heavier non-aromatics since they are moreselectively absorbed by the solvent phase and are present in highconcentrations in the hydrocarbon phase between flow conduits 4 and 5(relative to the heavy non-aromatics). Therefore, the extract phaseleaving extractor 3 through flow conduit 8 contains predominantly lightnon-aromatics as the Anon-aromatic portion. This latter example can beeconomically and efficiently carried out even with a single feed stockby introducing said feed into a splitter column, removing a light feedstock portion overhead and introducing it into the lower inter-mediatepoint in the extraction zone while removing a heavy feed stock portionfrom the bottom and introducing it into the middle intermediate point.The important point in all these cases is that the feed whosenon-aromatic components are more volatile in an extractive stripper isintroduced into a point closer to the extract withdrawal point thananother feed whose non-aromatic components are less volatile in theextractive stripper.

The extract phase leaves extractor 3 through ow conduit `f5 where itmixes `with additional lean solvent owing in ow conduit 9, if desired,and the total mixture flows through flow conduit 10' and into extractivestripper 11. The stripper is operated at elevated temperatures andintermediate pressures (relative to the extractor pressure) in order toremove substantially all of the nonaromatics, some of the water andaromatics and a small amount of the solvent overhead. This overheadstream is removed from stripper 11 through flow conduit 12 where itpasses through condenser 13 and into overhead receiver 14. The overheadstream is condensed and separated into two phases, one a water-solventphase and the other a hydrocarbon phase. This hydrocarbon phase iswithdrawn from receiver 14 through flow conduit 15 Where it is recycledto ow conduit 5 and eventually into extractor 3. Recycling this streamassures complete recovery of aromatics while also providing an additionsource of volatile non-aromatics to displace the less volatilenonaromatics from feed #2. The water-solvent phase settles in boot 16where it is withdrawn therefrom through ow conduit 17 and sent tosolvent recovery means not shown.

A bottoms stream consisting essentially of aromatic hydrocarbons andsolve-nt is withdrawn from the bottom of stripepr 11 and through fiowconduit 18 where a portion thereof ows through flow conduit 19, reboilerheater 20 and return to stripper 1t1. The remaining bottoms portion owsthrough ow conduit 21 and into solvent recovery column 22.

Column 22 is preferably operated at low pressures (relative to theextractor pressure) and elevated temperatures to separate the solventfrom the aromatics. The aromatics andsome of the water are removed fromcolumn 22 as a vapor and pass through ow conduit 23, condenser 24 andinto receiver 25. The overhead is separated into a hydrocarbon phasecomprising aromatics and a Water phase. The aromatics are withdrawn fromreceiver 25 through ow conduit 26 where a portion thereof ret-urns tocolumn 22 through flow conduit 27 as reflux -while the' remainingportion of aromatics is withdrawn through flow conduit 28 as netproduct. Generally, the net product is sent to a series of fractionatorsto recover the individual aromatics as substantially pure componentssuch as benzene, toluene, ortho-xylene, ethylbenzene, etc. A bottomstream consisting essentially of lean solvent is withdrawn from column22 through flow conduit 31 where a portion thereof flows through flowconduit 32, reboiler heater 33 and returns to column 22. The remainingbottoms portion iiows through flow conduit 34 where a fraction thereofflows into flow conduit 6 and into the upper-intermediate point ofextractor 3 while the other fraction ows into ow conduit 9 where itmixes with extract phase before entering stripper 11. A small amount ofdecomposition of solvent and other sludge forming reactions may occur inthe process and it is desirable to remove a small slipstream of leansolvent from flow conduit 34 through ow conduit 35 and regenerate thesolvent in equipment not shown in order to prevent the build up ofsludge. The regeneration preferably is done by rerunning the solvent andthereafter the regenerated solvent is returned to flow conduit 34 bymeans of `flow conduit 36. In addition suicient water is added to thelean solvent in equipment not shown to adjust the Water content of thelean solvent to the desired level. This is conviently done byintroducing the water into flow conduit 36.

Suitable feed stocks for the process of this invention comprises fluidhydrocarbon mixtures containing at least 6 carbon atoms per molecule.Feed #l is preferably selected in relation to feed #2 in order to attainthe desired non-aromatic displacement referred to hereinbefore. Apreferable combination is for feed #1 to be debutanized or 1depentanizedreformate (which is lean in napthenes) and for feed #2 to be adebutanized or depentanized hydrotreated pyrolysis liquid (which is richin naphthenes). Another preferable combination is for feed #1 to be alighter fluid hydrocarbon mixture such as in the C5 to C7 range and forfeed #2 to be a heavier fluid hydrocarbon mixture such as in the C7 toC10 range. It is readily apparent that many other suitable pairings offeed stocks may be used to attain the eicient stripping referred tohereinbefore and it is intended that these pairings be included withinthe scope of this invention.

Likewise, many solvents may advantageously be ernployed in the processof this invention providing they possess a solubility selectivity foraromatics over more saturated hydrocarbons. Preferable solvents includethe glycols, sulfolanes, sulfoxides, pyrrolidones, etc. Especiallypreferable solvents include diethylene glycol, various polyethyleneglycols, dipropylene glycol, various polypropylene glycols, dimethylsulfoxide, N-methyl pyrrolidone, saturated sulfolane (C4H8SO2),Sulfolene (CaHsSOz) etc. It is also within the scope of this inventionto add a polar diluent to the solvent such as water to further improvethe solvents selectivity for aromatic hydrocarbons. Preferably, thepresent solvents contain a small amount of water dissolved therein (fromabout 0.5% to about 20%) to increase the selectivity of the overallsolvent phase for aromatic hydrocarbons over non-aromatic hydrocarbonswithout reducing substantially the solubility of the solvent phase foraromatic hydrocarbons over non-aromatic hydrocarbons. The presence ofwater in the solvent composition furthermore provides a relativelyvolatile material therein which is distilled from the extract phase inthe extractive stripper 'which aids in vaporizing the last traces ofnon-aromatic hydrocarbon from the extract phase by steam stripping.Depending upon the particular solvent employed, water is added theretoin concentrations of from about 0.1% to about 20% by weight.

The extractor is operated at elevated temperatures and at a sufficientlyelevated pressure to maintain the feed stocks, recycle stream andsolvent in the liquidphase. Suitable temperatures are within the rangeof from F. to about 400 F. and preferably from about 150l F. to about300 IF. Suitable pressures are withinthe range of from about atmosphericpressure up to about `400 p.s.i.g. and preferably from about 50 p.s.i.g.to about 150 p.s.i.g. Suitable contacting means in the extractor vesselto efficiently contact the solvent phase with the hydrocarbon phaseinclude sieve decks, baffles, rotary discs, packing, etc.

The extractive stripper is operated at moderate pressures and suicientlyhigh reboiler temperatures to vaporize all the non-aromatichydrocarbons, and some of the aromatics, water and solvent. .Thestripper is preferably operated at lower pressures than the extractor,preferably from a slight vacuum up to about p.s.i.g. The reboilertemperature is dependent upon the composition of the feed and thesolvent.

The solvent recovery column is preferably operated at lower pressuresthan the stripper and sufficiently high temperatures to vaporize thearomatic hydrocarbons without thermally decomposing the solvent. In manycases, moderate to high vacuum must be employed in order to maintain asufficiently low reboiler temperature to avoid thermal decompositionwhile permitting vaporization of the aromatics. Pressures may be fromabout 100 mm. mercury absolute to moderate superatmospheric pressures inthe order of 20 p.s.i.g.

The apparatus employed in the process of this invention may be anyconventional convenient type known to those skilled in the art. Forsimplicity the drawing does not show all the pumps, tanks, heatexchangers, valves, bypasses, vents, preheaters, coolers, controlvalves, means for actuating control valves and other auxiliaries thatmay be necessary for the proper operation of the process but theinclusion of which will be evident to those skilled in the art.

The following example is presented to further illustrate the process ofthis invention but it is not intended to limit the invention to thematerials used or the operaing conditions disclosed therein.

A rst feed consisting of a reformate in the C6-C8 carbon number range isintroduced into flow conduit 5 (see the drawing) at a rate of 100moles/hr. whereupon it flows into the lower intermediate point (Ma ofthe height from the bottom) in extractor 3. The reformate containsapproximately 50 volume percent aromatics, 46 volume percent paraiiinsand 4 volume percent napthenes. A second feed consisting of ahydrotreated pyrolysis gasoline in the C6-C8 carbon number range isintroduced into flow conduit 4 at a rate of 1000 moles/hr. whereupon itflows into the middle intermediate point in extractor 3 (1A of theheight of the extractor). The pyrolysis gasoline 'contains approximately65 volume percent aromatics, 30 volume percent naphthenes and 5 volumepercent paraflins. Saturated sulfolane (C4H8SO2) is employed as theorganic solvent for this example. Lean solvent consisting of sulfolanecontaining about 5% (mole) water is introduced into the upperintermediate point in extractor 3 through flow conduit 6 at a rate of600 moles/hr. A recycle stream owing through ow conduit 15 is introducedinto extractor 3 at a rate of 140 moles/hr. The extractor is maintainedat a pressure of 100 p.s.i.g. and a temperature of 200 F. A raflinateproduct is withdrawn from the upper point in extractor 3 through ilowconduit 7 at a rate of 86 moles/hr. An extract stream is withdrawn fromthe lower point in extractor 3 through ow conduit 8 at a rate of 854moles/hr. where it contacts an additional 200 moles/hr. of lean solventflowing through flow conduit 9. The total mixture is introduced into owconduit 10 where it flows into stripper 11.

Stripper 11 is operated at a top pressure of 13 p.s.i.g., a toptemperature of about 225 F. and a bottom temperature of about 350 F.These conditions allow essentially complete vaporization of thenon-aromatics in stripper 11. The overhead vapors from stripper 11 arecondensed and settle into receiver 14. The hydrocarbon phase separatestherein and is recycled through flow conduit 15 at a rate of about 140moles/hr.

The stripper bottoms consisting essentially of aromatic hydrocarbons,organic solvent and a small amount of water is introduced into solventrecovery column 22. Column 22 is operated at a top pressure of about 200mm. mercury absolute, a top temperature of about 150 F. and a bottomtemperature of about 350 F. The aromatics are removed overhead in column22 and the organic solvent is removed out the bottom of column 22.Sufficient water is added to this organic solvent to bring the waterconcentration back to about 5%. The aromatics overhead is fractionatedinto the individual compounds, its purity is analyzed and a materialbalance is calculated. The purity of the benzene and toluene is at leastnitration grade and the purity of the C8 aromatics is above 99.5% Therecovery of benzene is about 99%, the recovery oi toluene is above 98%and the recovery of C8 aromatics is above 95%. It is expected that hadthe feeds been introduced into the same point in the extractor thepurity of the aromatics at the same operating conditions would have beenlower.

I claim as my invention:

1. A process for the separation and recovery of aromatic hydrocarbonsfrom at least two hydrocarbon feeds including a rst feed comprisingaromatics and a nonaromatic portion rich in parains and a second feedcomprising aromatics and a non-aromatic portion rich in naphthenes,which process comprises:

introducing a lean solvent into an upper intermediate point in theheight of an extraction zone;

introducing said second feed into a middle intermediate point in theheight of said extraction zone; introducing said first feed into a lowerintermediate point in the height of said extraction zone;countercurrently contacting the solvent with said feeds in theextraction zone; removing a hydrocarbon raffinate phase consistingessentially of non-aromatics from an upper point of said extractionzone;

removing a solvent extract phase enriched in aromatics content from alower point of said extraction zone; introducing the extract phase intoan extractive distillation column and stripping essentially all of thenonaromatic hydrocarbons from the extract phase; and separating thearomatics from the solvent.

2. The process of claim 1 further characterized in that the solventcomprises sulfolane.

3. The process of claim 2 further characterized in that the leansulfolane solvent contains up to by volume of water.

4. The process of claim 1 further characterized in that the solventcomprises sulfolene.

5. The process of claim 1 further characterized in that the solvent isselected from the group consisting of the polyethylene glycols and thepolypropylene glycols.

6. The process of claim 1 further characterized in that the solventcomprises dimethyl sulfoxide.

7. The process of claim 1 further characterized in that the solventcomprises N-methyl pyrrolidone. Y

8. A solvent extraction process for the separation and recovery ofaromatic hydrocarbons from reformate feed and from a hydrorenedpyrolysis feed, said reformate feed comprising aromatics and anon-aromatic portion rich in parains and said pyrolysis feed comprisingaromatics and a non-aromatic portion rich in naphthenes, which processcomprises:

introducing a lean solvent into an upper-intermediate point in theheight of an extraction zone;

introducing the pyrolysis feed into a 4middle-intermediate point in theheight of said extraction zone; introducing the reformate feed into alower-intermediate point in the height of said extraction zone;countercurrently contacting the solvent with the feeds in the extractionzone; removing a hydrocarbon raffinate phase consisting essentially ofnon-aromatics from an upper point in the height of said extraction zone;

removing a solvent extract phase enriched in aromatics from a lowerpoint in the height of said extraction zone;

introducing the extract phase into an extractive distillation column andstripping essentially all of the non-aromatic hydrocarbons from theextract phase; and separating the aromatics from the solvent.

9. The process of claim 8 further characterized in that the reformateand the pyrolysis feeds are within the C6 to C10 carbon number range.

10. The process of claim 1 further characterized in that the overheadhydrocarbon material from the extractive distillation column is recycledto said lower-intermediate point.

11. The process of claim 1 further characterized in that additional leansolvent is mixed with the solvent extract phase before the extract phaseis introduced into the extractive distillation column.

References Cited UNITED STATES PATENTS 2,365,898 12/1944 Morris et al.260-674 2,407,820 9/1946 Durrum 260-674 2,770,663 11/1956 Grote 260-5742,886,610 5/1959 Georgian 260-674 3,037,062 5/1962 Gerhold 260-6743,200,165 8/ 1965 Eisenlohr et al 260-674 DELBERT E. GANTZ, PrimaryExaminer.

C. E. SPRESSER, JR., Assistant Exdmner.

U.S. C1. X.R. 208-321, 325

